Thermal control of metabolic cold defence in pigeons columba livia
Metabolic rate (MR) and various body temperatures were recorded in pigeons Columba livia during gradual lowering of the ambient temperature (Ta) and during cold exposure (-10 degrees C) for an entire circadian cycle. The aim was to study how changes in MR accorded with the observed body temperature displacements and our knowledge of regional cold sensitivity. When Ta was lowered from 28 to -10 degrees C, the threefold increase in MR was accompanied by a significant increase in deep-body and spinal cord temperatures. Cold exposure also resulted in a distinct rise in breast skin temperature, whereas the skin temperatures of the neck, back and leg decreased significantly. Thus, during acute cold exposure of pigeons, peripheral thermosensors seem to have the primary effect in mediating metabolic cold defence, while internal thermosensors are apparently of minor importance. By using differential weightings of various skin areas, cold-sensitivity values of between -3.9 and -14. 7 W kg-1 degrees C-1 were estimated. These values indicate a much higher skin thermosensitivity of birds than has been previously suggested. When cold-exposed for an entire circadian cycle, the pigeons usually maintained a low leg skin temperature. However, during these experiments, the pigeons showed regular distinct increases in leg skin temperature, obviously due to cold-induced vasodilatations (CIVDs). The flushing of cold peripheral blood through the body in connection with these CIVDs resulted in a decrease in deep-body temperature (Tb), to which the pigeons responded with a distinct increase in MR. From the concomitant changes in MR and Tb, a total body cold sensitivity of -8.0 W kg-1 degrees C-1 was calculated, a value that is close to the previously published cold-sensitivity value of pigeons. This shows that, at least under some naturally occurring conditions, internal thermosensors will provide a significant error signal in the control of metabolic cold defence. Furthermore, the results of our study show that the relative importance of various thermosensitive sites in the control of metabolic cold defence is contextual.
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